1. Field of the Invention
This invention relates to a liquid crystal display device and more particularly, to an active matrix color liquid display device wherein a high contrast ratio can be obtained.
2. Description of the Prior Art
In known liquid crystal display devices, there is provided a means for lowering a contrast ratio owing to the leakage of light from portions other than picture elements when a totally black display is made in the normally white display mode. For instance, as shown in FIG. 5, portions other than a color filter layer 1, which is a picture element, are covered with a metal film or black matrix layer 2 made of a black resin and serving to absorb light in such a way that the picture element and the film or metal 2 are partly overlapped while taking an exact registration of the picture element into account.
FIG. 5 is a view, partially in section, of a liquid crystal display (LCD) panel which makes use of a substrate on which a polysilicon thin film transistor (poly-Si TFT) is formed as an active switching element. This structure is briefly described. A pair of quartz substrates 3a, 3b which are in face-to-face relation are provided, between which a liquid crystal layer 4 is sandwiched. There are provided between the quartz substrate 3a and the liquid crystal layer 4 a color filter layer 1, a black matrix layer 2 and a common electrode 5. On the other hand, there are provided between the quartz substrate 3b and the liquid crystal layer 4, for example, a thin film transistor (TFT) 6 used as an active switching element, a first PSG layer 7 and a second PSG layer 8, both used as a layer insulating film, and a picture element electrode 10 made of ITO. The TFT 6 drives the picture element electrode 10. The quartz substrates 3a and 3b have, respectively, polarizing plates 11a, 11b for controlling light emitted from or toward outside. Light 13 is passed through a back light module 12. In FIG. 5, shaded regions A of the liquid crystal layer 4 are overlapped regions of the picture element (color filter layer) 1 and the black matrix layer 2. Indicated at 28 is an A1 signal line.
When the LCD device having such an arrangement as stated above is displayed as white over the entirety thereof, light is absorbed at the black matrix portions A. This eventually leads to a dark display over the entire picture thereof.
This phenomenon becomes more serious when the high definition of the device is more advanced. This is because the ratio of the black matrix region to the picture element region is increased.
Japanese Laid-open Patent Application No. 62-94826 discloses a liquid crystal display device which includes a substrate on which active switching elements are formed, and another substrate having a color filter at a side opposite to the first-mentioned substrate, on which micro-lenses are formed, by which the picture is made bright with an improved contrast.
However, if a black matrix layer is provided between the color filters in the prior art devices, any incident light is not passed from the black matrix layer portion, resulting in a poorer light transmittance. In addition, the fabrication process undesirably becomes complicated.